The present invention generally relates to improvements in a high-frequency heating apparatus for heating food, liquid or the like by a so-called dielectric heating, and more particularly, to a magnetron feeding apparatus and a method of controlling the magnetron feeding apparatus which is adapted to effect a feeding operation to a magnetron by an inverter circuit using a semiconductor switch and a high-tension transformer.
Generally, a magnetron feeding apparatus shown in FIG. 1 is adapted to convert a DC current 2 from a power supply portion 1 into high-frequency power 5 by an inverter circuit 4 using a semiconductor switch 3 such as a transistor, and furthermore to convert into high-tension power 7 by a high-tension transformer 6 to feed the power to a magnetron 8.
Also, a method of controlling the feeding power comprises steps of detecting the input current from a commercial power supply 9 by an input detecting portion 10, and forming the difference 12 between its value and a reference signal 11 by an error amplification circuit 13; a driving circuit 14 generates a driving pulse 15 corresponding to the difference 12 so as to turn the semiconductor switch on and off, so that the input current is controlled to be a given value.
However, as most of the power to be fed to the magnetron 8 is consumed only by a heater 8.sub.H before the magnetron 8 starts its oscillation, the power feeding operation will be approximately one-seventh of feed power at the steady-state time when the magnetron 8 is oscillating.
Namely, if the input current is controlled to the given value in a period before the magnetron 8 starts its oscillation, the excessive power is applied on the heater 8.sub.H and also the excessive voltage is applied upon between the anode 8.sub.A and the cathode 8.sub.K, thus resulting in the life service of the magnetron 8 being shorter because of discharging, deteriorated insulation, etc.
The reference signal 11 is set to a value smaller at the steady-state time before the magnetron 8 starts its oscillation so as to be controlled to small input current which is proper to the heater heating operation. By using a timer 16, the reference signal 11 is switched to the value at the steady-state time after the lapse of the time sufficient enough for the magnetron 8 to start its oscillation, so that the input current may be controlled to a value at the steady-state time.
As time required for the oscillation to be started is two seconds when the magnetron 8 is kept warm, four seconds when it is kept cold, the time for the timer 16 is set as five seconds for safety.
As the wave output of the magnetron 8 is not produced for five seconds, a wasteful time of 5-2=3 seconds is consumed when the magnetron 8 is kept warm, while a wasteful time of 5-4=1 second is consumed when the magnetron 8 is kept cold, with inconvenience that high-speed cooking capability which is characterized by the electronic range is deteriorated.